1. Field of the Invention
The present invention relates to a pressure relief check valve, and, more particularly, to a valve assembly including a poppet with a fluid metering stem to regulate the passage of fluid through the valve, such as during a cracking mode and other open valve conditions.
2. Description of the Related Art
Conventional relief valves typically employ a check ball that serves both as the valve mechanism and seal. During operation, after the cracking pressure has been reached, the check ball will rise under the influence of the fluid pressure and create space about the ball surface to enable fluid to flow past and exit the valve. In typical designs, the intake and exhaust ports will be positioned at opposite sides of the check ball in a diametrically opposing configuration. Accordingly, fluid will flow over and around the check ball as it traverses the pressure relief path defined by the valve.
Conventional valve designs possess various drawbacks, such as susceptibility to leaking because of particulate contamination, a large spread for cracking pressure points to reseat pressure points, a high flow rate at full flow condition, and poor consistency of operating points.
Regarding the spread for cracking pressure points to reseat pressure points in a check ball valve design, the fluid mechanics relating to its operation do not permit the valve to exhibit the momentary open/close characteristic desired for the cracking state. After the point of cracking has been reached in a check ball design, the surface area that experiences the hydraulic force induced by the fluid pressure (both statically and dynamically) increases as the check ball becomes unseated and moves away from the valve seat. Standard poppet valve designs also exhibit the same behavior vis-à-vis the increase in the pressure bearing surface area after cracking pressure has been achieved.
This variation in the surface area as the valve opens is an inherent feature of standard valve designs that occurs normally during valve operation. In check ball designs, for example, it is seen that the increased surface area exposed to the fluid pressure corresponds to the increased amount of surface area of the spherical check ball that the fluid must travel along to reach the relief port. Therefore, assuming that the pressure differential remains the same across the valve, the fluid pressure will exert a force upon the check ball that is directly proportional to the area over which the force is acting. This relationship is readily seen in the definition of pressure, namely, force per unit area.
As the surface area exposed to fluid pressure increases, there will then be a corresponding increase in the pressure-related force that acts upon the ball. This increase in the force exerted upon the ball acts to enlarge the crack created at the interface between the check ball and valve seat and thereby admit more fluid into the exhaust port. The force continues to increase as the crack widens and more surface area of the check ball is exposed to the fluid pressure. The force will increase, unless otherwise dampened or impeded, until a full open condition is reached. It may happen, then, that even though only a cracking action is desired, a continuous flow condition may ensue that results in unwanted and prolonged loss of fluid.
The rising force level not only acts at cross-purposes to the desired cracking behavior (i.e., a momentary on/off response), but imposes a limitation on the valve design in terms of its responsiveness to pressure changes aimed at reseating the valve. The automatic tendency of the check ball (at constant pressure) to continue its displacement away from the valve seat in apparent self-perpetuating fashion means that any attempt to reseat the check ball will require a not insignificant decrease in pressure to counteract or otherwise compensate for the increased surface area exposed to fluid pressure.
Generally, the fluid pressure will have to be reduced to a level that enables the force of the closing mechanism (e.g., spring) to exceed the oppositely directed force established by the fluid pressure. This results in the large spread between cracking pressure point and reseat pressure point.
The required pressure drop will likely diminish or at least temporarily adversely affect the throughput of the fluid delivery system that uses the check ball valve. In a fuel dispenser, for example, this diminished performance would be an unacceptable operating factor that a site operator would not want a customer to experience during refueling.
The force increase attending check ball valves would typically be addressed by monitoring the condition of the valve and incorporating a functionality that quickly reduces the valve pressure after the cracking condition has been detected. In this manner, the valve is not given a chance to rapidly advance towards a full open condition, which otherwise would happen if the pressure differential across the valve remained constant. This additional functionality, however, adds complexity and cost to the valve design.
The increase in surface area (and the corresponding increase in force acting against the check ball) also makes it difficult to implement a controlled level of pressure relief and/or fluid release. This control aspect would involve a metering behavior that can be precisely regulated to allow gradual and progressive changes in the metered fluid flow in response to pressure changes. However, due to the automatic rise in pressure-related force as the check ball unseats following the onset of the cracking point, a conventional valve may reach its full open condition without little or any variation in the fluid pressure.
There exists, then, a narrow spread between the cracking reseat point and the full flow reseat point in check ball valve designs. More particularly, the pressure range between cracking point and a full open condition is relatively small. It would be more preferable that the valve advance from cracking to full open in response to a progressive increase in the fluid pressure, i.e., a relatively wider pressure range.
Various other drawbacks are evident with valve constructions using a check ball. For example, check ball designs do not have a feature that limits the compression of the bias spring. Check ball designs also can be affected by misalignment errors as the normally displaced ball drifts sideways and is pushed away from axial registration with the valve seat. This causes a single point contact between the ball and seat, resulting in a cantilever effect that lowers the reseating pressure points.